How many Hiroshimas does it take to describe climate change?

Canadian rock legend Neil Young is worried about the environmental impacts of extracting oil from the tar sands near Fort McMurray, Alberta. “The fact is, Fort McMurray looks like Hiroshima,” Young said at a recent event for the National Farmers Union in Washington, DC, where he warned that carbon emissions are causing climate chaos for farmers. Some critics called Young’s reference to the devastating World War II nuclear attack irresponsible and unwarranted but most focused on extolling the virtues of Fort McMurray rather than those of the tar sands that lie beyond city limits.

Young isn’t the only climate campaigner to go nuclear in recent months. At the Climate Action Summit in June, John Cook, a research fellow on climate communication at the University of Queensland’s Global Change Institute, said “our planet has been building up heat at the rate of about four Hiroshima bombs every second—consider that going continuously for several decades.” The headlines seized on Cook’s analogy: “Climate change like atom bomb,” The Sydney Morning Herald reported, and a number of other newspapers followed suit.

Cook was merely reformulating numbers that climate scientist James Hansen (a member of the Bulletin’s Science and Security Board) articulated during a TED talk in March 2012: Displaying an image of the Hiroshima mushroom cloud, Hansen said that the excess energy building up in the Earth’s oceans and other heat reservoirs is “equivalent to exploding 400,000 Hiroshima atomic bombs per day, 365 days per year.” With those words, a meme was born.

Climate skeptics pounced on the comparison, calling it a “ridiculous scare story” that is intentionally manipulative. There are really three separate issues here, though: One is a question of the comparison’s accuracy, another of its effectiveness, and third of its appropriateness.

Is the comparison accurate? Hansen based his comparison not on the instant mass death atomic bombs can cause but on their explosive yield: the amount of energy they release when detonated. Bomb yield, usually expressed in kilotons of TNT, can be converted to an equivalent number of calories for direct comparison with Earth’s rising heat content.

Heat is building up in the entire Earth system, especially the oceans, because emitting carbon dioxide and other greenhouse gases “is like throwing another blanket on the bed,” Hansen explained. “It reduces Earth’s heat radiation to space, so there’s a temporary energy imbalance; more energy is coming in than going out.” The total imbalance, about 0.6 watt per square meter of the Earth’s surface, “may not sound like much,” Hansen said, “but when added up over the whole world it’s enormous.”

Skeptic Anthony Watts dismissed this argument by pointing out that the excess energy “hardly registers a blip” compared with the total amount of energy received from the “the biggest fusion bomb in our solar system, our sun.” The imbalance is only enough to illuminate one square meter with 1/100th the power of a 60-watt incandescent light bulb. Sounds insignificant, doesn’t it?

But even Watts conceded that Hansen and Cook didn’t get the numbers wrong. Moreover, both Hansen and Cook emphasized that the cumulative nature of the energy imbalance is what makes it significant. As one scientist-blogger pointed out, if you lived in a typical-size house with 0.6 watt per square meter of heat energy never leaving the premises, the temperature inside your house would rise by about 4.5 degrees per day—and boil you within a month. That’s a blippin’ problem.

Is the comparison effective? Skeptical blogger Donna Laframboise deemed the Hiroshima meme a “media fail,” but from Cook’s perspective it clearly was not: His pronouncement made headlines around the world, garnering even more attention than Hansen’s TED talk. (It’s telling that Laframboise’s blog post attacked Cook’s climate creds, rather than his argument.)

Talk of atom bombs definitely makes journalists sit up and take notice, but what about the general public? Nothing is scarier than a nuclear attack, but does scaring people prompt them to take action? Or could it be counterproductive?

Cook said he used the bomb analogy after reading the book Made to Stick by Chip and Dan Heath. Misinformation about climate change is what the Heath brothers call a “sticky” idea, Cook explained, so he took their advice to “fight sticky ideas with stickier ideas.” Climate scientists often speak in complex, abstract, dry language, whereas sticky messages are simple, concrete, unexpected, and emotional—but also credible.

Is the comparison appropriate? The Hiroshima meme is not only credible but also justifiable, for several reasons. First, comparing climate change with an atomic bomb reminds people that climate change is a manmade phenomenon. While humans are not entirely responsible for the increase in global temperature since pre-industrial times, they are to blame for the energy imbalance that Hansen talked about.

Second, the Hiroshima meme frames climate change as something catastrophic. The analogy’s primary purpose is to explain the magnitude of the climate threat and to spur action, not to terrify people. (It’s worth mentioning that Hansen is pro-nuclear when it comes to power generation.) The most common reaction to the meme is surprise, not fear: “Wow. I had no idea.”

Sure, the mushroom cloud has become a cliché image for conveying disaster. It’s an apt one in this case, though. The picture of a mushroom cloud over Hiroshima is buried deep within America’s national consciousness, and awareness of the bomb’s impacts is what ultimately led to international treaties aimed at preventing any future use of nuclear weapons. To avert another tragedy of global proportions, the world’s superpowers must now lay down their fossil fuels as well.

Climate change won’t destroy future generations as instantaneously as Little Boy incinerated the people of Hiroshima, but continuing with business as usual guarantees that millions of people will die as a result. If the Hiroshima meme “trades on human tragedy to make an illustrative point,” as one blog commenter complained, it does so with abundant moral justification.

Hansen and Cook did not argue that global warming causes as many deaths as an atomic bomb, but they could have. Climate change is entirely capable of causing mass death—not just by increasing the frequency and severity of extreme weather events such as heat waves and floods, but also by creating conditions that make it easier for diseases to thrive and food crops to fail. Climate change is already killing an estimated 400,000 people annually, mostly children. That’s more than the total population of Hiroshima in 1945.

These deaths are happening now, not in some distant future, and not just in developing countries. Between 1999 and 2009, an average of 658 heat-related deaths occurred in the United States alone, according to the Centers for Disease Control and Prevention. An international team of scientists who studied the extreme weather events of 2012 recently estimated that events like the July 2012 US heat wave are now likely to occur four times as often as they did when greenhouse gases in the atmosphere were at preindustrial levels.

Under these circumstances, scientists (and musicians) should feel no shame about shouting fire in a crowded theater. It’s far better to sound the alarm now than to wait until the theater is engulfed in flames, as climate skeptics would have it. The current situation warrants strong language—and strong images.

Pictures of ordinary humans might be even more effective at conveying the dangers of climate change than a mushroom cloud. Recent social science research suggests that seeing other humans in peril can move people to action. A photo of a naked girl burned by napalm was far more powerful in changing the public mindset about the Vietnam War than any images of exploding bombs. Where, then, are the photos of the 37 American children killed by hyperthermia this year? And how many pictures of innocent victims and mushroom clouds will it take before a light bulb goes on in enough heads to turn things around?